Direct and indirect mechanisms behind successful biomanipulation

Abstract

Lake Vesijärvi is a relatively large (length 25 km; total area 110 km 2 ), shallow (mean depth 6 m), but stratified lake in southern Finland. The Enonselkä basin (26 km 2 ), surrounded by the city of Lahti, received its sewage effluent, and changed from a clear water basin with flourishing fisheries from the 1940-50s to one of the most eutrophic lake systems in Finland thereafter. In 1976, the sewage effluent was diverted, resulting in a temporary recovery of water quality. However, in the 1980s, massive surface scums of cyanobacteria degraded the water quality and arrested the recovery of the lake. A restoration strategy providing an ecologically sound basis for the management of the lake was initiated in 1987. This strategy involved biomanipulation (mass removal of coarse fish) together with conventional pollution control measures on discharges to the lake. Biomanipulation was chosen instead of much more expensive chemical and/or technical methods, such as chemical treatment or dredging of the profundal sediment. The large-scale biomanipulation trial was carried out in the Enonselkä basin during 1989-93. Following the mass removal of coarse fish (1000 metric tons of fish; mainly roach and smelt), the biomass of cyanobacteria collapsed concomitantly with a decline of total phosphorus concentration from 45 to 35 mg P m −3 , and with an increase of Secchi depth from 1 m to 3.5 m. These observed improvements in the water quality were matched with a large decline in roach-mediated phosphorus movement from littoral to pelagial, from 100 mg P m −2 in 1989 to 15 mg P m −2 in 1993. Year-to-year variation within the littoral communities, and in the recruitment of fish, could in this way cause large oscillations in the whole ecosystem. The involvement of local people (fishermen, farmers etc.) in controlling non-point nutrient loading and fish stock development, is of prime importance for the long term success of lake restoration.

Response of zooplankton to nutrient enrichment and fish in shallow lakes: a pan‐European mesocosm experiment

“…, 1998; Jeppesen et al. , 1998b; Kairesalo et al. , 1999) and nutrient availability (Hansson, 1992; Persson et al.…”

Identification of a biomanipulation‐driven regime shift in Lake Vesijärvi: implications for lake management

“…Previous studies of Lake Vesijärvi concluded that improved lake status was mainly due to direct effect of fish on nutrient concentrations and that trophic cascades were considered less important. Trophic responses were judged to result from diminished excretion and bioturbation by fish, leading to reduced fish‐mediated nutrient transfer from benthic and littoral habitats to the lake pelagic zone (Horppila et al ., ; Hansson et al ., ; Kairesalo et al ., ). The decrease in TP at the beginning of restoration supports these conclusions.…”

Sediment resuspension: rescue or downfall of a thermally stratified eutrophic lake?

“…Originally, a clear water basin underwent eutrophication due to sewage effluent that emanated from the city of Lahti that surrounds the lake basin (Keto & Sammalkorpi, 1988). In the 1980s, massive cyanobacteria blooms degraded the water quality of the basin and therefore biomanipulation measures were implemented, along with diffuse load reduction, as restoration actions in the late 80s and early 90s (Kairesalo et al, 1999). Water quality improved, but within 10 years, cyanobacteria blooms had occurred again (Keto et al, 2005).…”